Semiconductor fabrication uses a process known as lithography to form sub-micron features on a substrate. One type of lithography known as photolithography is based on exposure of a photosensitive resist to light radiation that is focused through a patterned mask. A layer of the resist material is formed on a semiconductor wafer. The radiation is focused on the surface of the resist to project the mask pattern onto the resist. Portions of the resist that are exposed to the radiation are altered in a way that either makes them susceptible to removal (in the case of a positive resist) or resistant to removal (in the case of a negative resist). Developing the resist transfers the mask pattern to the resist and allows a pattern of material to be removed from or deposited on the substrate through a pattern of openings in the resist.
Another type of lithography referred to as direct-write lithography uses a narrow energetic beam such as a laser beam or a beam of electrons. The beam shines on the resist while the wafer moves relative to the plane of incidence of the beam. As the wafer moves, the beam is turned on and off to expose the resist in a predetermined pattern.
Lithography systems used in semiconductor manufacturing often include a system of sensors and associated electronics and software to sense wafer position during lithography. Such systems are ultimately responsible for maintaining the registration of the printed pattern with respect to the wafer during lithography. In prior art lithography systems a wafer is typically aligned before the exposure process starts. The process relies on maintaining the pre-exposure alignment through the completion of the exposure process by means of precision control of a stage to which the wafer is mounted. Such systems can account for deviation of the pattern registration with counteracting rigid body motions of the stage. Unfortunately, a wafer on the stage may move with respect to the stage during the exposure process. Furthermore, the wafer may be subject to distortions in its shape during the exposure process. Both of these are not taken into account by prior art wafer alignment systems.
Prior art wafer alignment systems may take rigid body motions of the wafer into account, e.g., by solidly attaching the wafer to the stage. Prior art lithography systems may also rely on field by field alignment (mapping) before the exposure process to map wafer distortions. Such field-by-field mapping takes into account wafer distortion when the wafer motion (i.e., the stage motion) is stopped. However, field-by-field alignment does not take into account distortions occurring during movement of the wafer as the exposure takes place.
It is within this context that embodiments of the present invention arise.